Network Tick-Tack-Toe

Assigned homework 24 January 2003.

Goal

Write a tick-tack-toe program that can be played across the network, using the client-server paradigm, in Java (C/C++ also permitted).

Step-by-step

1. The Chat
   You have been given examples of Java network programming which should allow you to create a client-server chat program: two applications which allow you to type back and forth across the the network. (See Java by Example handout.) Your first step is to get these working.
2. The Protocol
   We need to define the messages which will be sent between client and server. This is a very important step, and we need to be very accurate. There are four messages.
   • The first message and the first message sent upon client-server connection is a banner from the server to the client. Something like TTT V0.1 is sufficient.
   • Their are three other message types. They are identified by the first letter of the message: N, B, or Q.
   • The N and Q messages are just that single letter. The B message is of the form B********* where the nine stars are replaced by either X, O, or -.
   • The client drives the conversation. Each message from the server, except the banner, is a response to a message from the client. This simplifies coding. From the client's point of view, the loop is: write, read, think, write, read, think, and so on.
   • Except for the banner, the server reacts to client's messages and must respond to each message. This simplifies coding. From the server's point of view, the loop is: read, think, write, read, think, write, and so on.
   • The B message: The board update message is the most important message. It describes the current board. It is sent either from server to client or client to server. Example: suppose X is in the upper left and lower right squares and O is in the middle square. The message sent will be BX---O---X.
     Note well: It is not the responsibility of the protocol that the game is played in any way correctly. It is only cares that these messages are syntacticly correct. This protocol is our implementation of the OSI Session and Presentation layers. The 7-th layer will insure that new and old board positions agree with the laws of play.
   • The Q message: The quit message ends the game and the communication. Parties must try not to simply hang up on each other, but should use a quit message to exit together. Either client or server can begin the quit process by sending a Q. However, if the client sends, the server must respond with a Q, and then exit. If the server sends first, the client does not respond, but both client and server exit. This is because the client expects a response for every message it sends. Neither client nor server can refuse a quit.
   • When the client exits, it exits the client application program. The server however only exits the connection and it goes back to the listening socket for the next connection.
   • The N message: The new game message is somewhat optional. It is probably most needed when the board update message received shows that the game has been won.
   • Suppose the client makes the winning move. Its B message requires a response. The server responds N offering a new game. The client can return a B message with its first move, or an N message offering first move back to the server, or an Q to request the server to terminate the connection.
   • Suppose the server makes the winning move. Its B message does not require a response, but protocol is that if the client wants to quit it should send an Q message and wait for an Q response. Or it can send a B message with its first move of a new game, or an N message offering first move to the server.
   • How the choice is made between the above options is not the responsibility of the protocol. The application software can be simple or sophisticated, negotiating a new first player or not.
   Get this protocol working after the chat is working. It consists of loops on the server and client code which interpret the messages and make reasonable responses. In particular, don't worry too much about correct game boards, rather that the rules of message interchange are followed.
3. Playing the Game
   Now add the application layer software which presents the board to the user pictorially, offers opportunities to move, resign or quit. Also the application layer can check new and old boards for correct play and check if the game has been won. If the game is won it can prompt the user whether it wants a new game, whether it wants to go first, or whether it would rather quit. Do this layer last. It is the least tricky and you can spend endless hours making the interface handsome, friendly and flexible.